Page 69 - Mechanics of Asphalt Microstructure and Micromechanics

62 Ch a p t e r Th r e e

a. A Sectional Image b. 3D Visualization
FIGURE 3.4 Sectional image and 3D image acquired through X-ray tomography imaging.

of different pixels (spatial location) can be obtained and visualized. Figure 3.4a presents
one of the sectional images (mapping of the attenuation coefficients on the spatial loca-
tions). By moving the specimen down a little (Figure 3.3), another sectional image can
be obtained, etc. Figure 3.4b presents the 3D visualization of 81 sectional images of an
AC specimen. Unlike a conventional picture, this visualization contains details of the
structure of the material.
Most current computed tomography (CT) systems include cone beam scanning,
and sometimes spiral scanning. While many of the asphalt researchers use industrial
CTs, some use medical CTs as well. Some more recent CT systems have reached resolu-
tions up to 50 nm with specimen size limitations of 50 micrometers.
In recent years, XCT has also been used in the characterization of soil, cement con-
crete, wood, and composite materials. The next few sections describe some of the ap-
plications in AC.

3.2 Fundamental Stereology Principles and Interpretation
Stereology is a science that abstracts 3D information from 2D observations. Major re-
sources come from Buffon Transform. A brief introduction to some of the fundamental
principles is presented. The selection of these principles is based on the understanding
that the geometric factors of interest to asphalt concrete researchers include the follow-
ing:
1. Volume fraction of the constituents (aggregate, binder/mastics, and air voids).
2. Local volume fraction of the constituents and their gradients.
3. Specific surface area of aggregates and film thickness.
4. Spacing among the different entities (voids and aggregates).

Some fundamental concepts are presented as follows:
Random—In exact statistical meaning, “randomness” means equal chances for any
individual member of a population. It is used in random sampling, or sampling without
conscious choice of location and orientation. The results of any of the two random sam-
plings are therefore independent.